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From:   Peter Gonda <pgonda@google.com>
Date:   Thu, 19 Aug 2021 15:00:25 -0600
Message-ID: <CAMkAt6oJcW3MHP3fod9RnRHCEYp-whdEtBTyfuqgFgATKa=3Hg@mail.gmail.com>
Subject: Re: [PATCH 1/2 V4] KVM, SEV: Add support for SEV intra host migration
To:     Marc Orr <marcorr@google.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Rientjes <rientjes@google.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
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> >
> > +static int svm_sev_lock_for_migration(struct kvm *kvm)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +       int ret;
> > +
> > +       /*
> > +        * Bail if this VM is already involved in a migration to avoid deadlock
> > +        * between two VMs trying to migrate to/from each other.
> > +        */
> > +       spin_lock(&sev->migration_lock);
> > +       if (sev->migration_in_progress)
> > +               ret = -EBUSY;
> > +       else {
> > +               /*
> > +                * Otherwise indicate VM is migrating and take the KVM lock.
> > +                */
> > +               sev->migration_in_progress = true;
> > +               mutex_lock(&kvm->lock);
> > +               ret = 0;
> > +       }
> > +       spin_unlock(&sev->migration_lock);
> > +
> > +       return ret;
> > +}
> > +
> > +static void svm_unlock_after_migration(struct kvm *kvm)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +
> > +       mutex_unlock(&kvm->lock);
> > +       WRITE_ONCE(sev->migration_in_progress, false);
> > +}
> > +
>
> This entire locking scheme seems over-complicated to me. Can we simply
> rely on `migration_lock` and get rid of `migration_in_progress`? I was
> chatting about these patches with Peter, while he worked on this new
> version. But he mentioned that this locking scheme had been suggested
> by Sean in a previous review. Sean: what do you think? My rationale
> was that this is called via a VM-level ioctl. So serializing the
> entire code path on `migration_lock` seems fine. But maybe I'm missing
> something?
>

Marc I think that only having the spin lock could result in
deadlocking. If userspace double migrated 2 VMs, A and B for
discussion, A could grab VM_A.spin_lock then VM_A.kvm_mutex. Meanwhile
B could grab VM_B.spin_lock and VM_B.kvm_mutex. Then A attempts to
grab VM_B.spin_lock and we have a deadlock. If the same happens with
the proposed scheme when A attempts to lock B, VM_B.spin_lock will be
open but the bool will mark the VM under migration so A will unlock
and bail. Sean originally proposed a global spin lock but I thought a
per kvm_sev_info struct would also be safe.